Many types of AC power supplies, such as electronic ballasts for fluorescent lamps, include a boost converter for providing operational benefits such as power factor correction and line regulation. In theory, a boost converter can provide an output voltage that is many times greater than the input voltage. However, in actual boost converters, output voltages of greater than about twice the peak input voltage are highly impractical because the peak currents and power dissipation in certain components (particularly in the boost inductor and the boost switch) become prohibitively high under such conditions.
In certain applications, it is often desirable to have a boost output voltage that is on the order of at least two to three times the peak of the AC line voltage. One such application is in electronic ballasts that include a boost converter followed by a high frequency inverter. Because the boost output voltage serves as the input voltage to the inverter, and since the amount of dissipative current that flows in the inverter can be reduced by increasing the inverter input voltage, a higher boost output voltage tends to enhance inverter efficiency. Unfortunately, a higher boost output voltage may at the same time degrade the efficiency of the boost converter.
"Voltage doubler" circuits have been used for quite some time to provide a DC output voltage equal to twice the peak value of the AC line voltage. Conventional voltage doubler circuits are structurally simple and energy efficient, but are incapable of providing the high degree of power factor correction that is usually required in many types of power supplies and electronic ballasts.
A number of attempts have been made in the prior art to fulfill the need for a power factor correcting converter that efficiently provides high voltage gain. In particular, several inventors have developed "boost voltage doubler" circuits that combine the power factor correction advantages of a boost converter with the efficiency and high voltage gain of a voltage doubler. Examples of such circuits are described in U.S. Pat. Nos. 5,383,109 and 5,502,630. Unfortunately, existing boost voltage doubler circuits have the significant disadvantage of requiring at least two power transistor switches. Consequently, the control circuit for turning the transistor switches on and off is quite extensive. Thus, the prior art circuits tend to be rather complicated and costly and are therefore unattractive for use in power supplies and ballasts for which low material cost and ease of manufacture are important requirements.
It is thus apparent that a need exists for a voltage converter that efficiently provides high voltage gain along with a high degree of power factor correction, and that has a structure that is considerably less complex and expensive than existing circuits. Such a circuit would constitute a significant improvement over the prior art.